I found this...maybe it will explain a little more.
If it's 2 females, one is trying to literally put her foot down to establish dominance if she's a female stepping on another female. First of all, not all females have brown ceres. Both of my female parakeets have white ceres. Male parakeets have blue, pink, and sometimes purple ceres. When a male puts his foot on the back of the female, he's getting ready to do her “birdie style", if you will, to mate. But if it's just stepping on tails, it's just normal parakeet behavior and it's like you pulling on your sister's ponytail. Just parakeet rivalry and teasing. Mine have stepped on the other one, trying to establish dominance, because the other one is fairly new and still a young one. But, after Cloudie steps on Sunryze's back, they bob their heads and chirp, so they're just playing. Nothing to worry about.